ui : group discover and selector components into folders
Move the ModelsDiscover components into ModelsDiscoverList/ and
ModelsDiscoverDetails/ subfolders (with ModelsDiscoverDetailsDownloadOptions/
nested inside), and the model selector components into a ModelsSelector/
subfolder. Rename ModelsDiscoverModelDetails* to ModelsDiscoverDetails* for
consistency, add per-folder index.ts barrels, and keep shared leaves
(ModelId, ModelBadge, ModelLoadHighlight, ModelsDiscoverAvatar,
DownloadProgressBar) at their folder roots.
